(1) Field of the Invention
This invention relates to recycling of fluids which have been polluted or contaminated. The majority of the fluids treated according to this invention are used in natural gas treating plants, primarily Triethylene Glycol (also called TEG). In addition, ethylene glycol and diethylene glycol, which find use as anti-freeze, also accumulate hazardous material and are recycled by this process.
Operators of natural gas dehydration and treatment plants will have ordinary skill in this art.
(2) Description of the Related Art
Natural gas flowing from wells may contain water vapor, liquid water, brine solution, heavy and light hydrocarbons, and particulate matter such as sand, pipeline scale and rust. If these elements remain in the gas stream they will cause numerous problems in the pipeline and processing equipment. Treating natural gas will commonly occur at individual wells, gathering systems, compressor stations, distribution stations, and gas processing units. Lowering the water content of natural gas at these sites will prevent the clogging of pipelines due to hydrate formation. Hydrate formation will occur with the combining of water and natural gas molecules. Hydrates will block valves and flowmeters. Hydrates will also accumulate at low points in the pipeline. Hydrates will decrease pipeline efficiency and cause shutdowns. The hydrates will also increase erosion and corrosion. To prevent the formation of hydrates, natural gas must be dehydrated at the aforementioned sites prior to reaching the pipeline.
Processes for removal of entrained water vapor and other contaminants in natural gas are well known. The most common process for the removal of water in natural gas is glycol dehydration. The process is done in a natural gas dehydration plant. Where TEG is the preeminent desiccant used in the dehydration process. TEG offers the following advantages: 1) ability to absorb large amounts of water, 2) relatively low solubility of valuable gas constituents, 3) chemical stability, 4) easy to regenerate, and 5) low cost. Before treating natural gas with glycol dehydration, the wet natural gas passes through an inlet separator where water droplets, liquid hydrocarbons, entrained sand, rust and so forth are separated out of the gas stream. The wet natural gas then flows to the absorber or contacting tower where it is interfaced with the TEG. The wet natural gas will enter the contacting tower at the bottom where it will flow upward, while lean TEG, free of water, will enter the top of contacting tower where the TEG flows downward. The counter current flow will aid the lean TEG in absorbing most of the water contained in the natural gas. The natural gas stream leaving the tower is said to be dehydrated. The TEG leaving the tower is called rich TEG. The rich TEG is passed through sock filters to remove any particulate matter picked up by the TEG. The rich TEG then flows to a reboiler or regenerator where it is heated to drive off any of the absorbed water contained in the rich TEG solution. After the TEG has been regenerated, it is then recycled-for reuse in the dehydration system. The TEG is recirculated numerous times per hour through the entire dehydration system(absorber tower and regenerators).
In normal use a supply of TEG will run for several months before it gets so laden with impurities that it is no longer efficient to continue use. Many of the contaminants are hazardous materials requiring expensive limitations upon their disposal. This refining unit is preferably mounted on a trailer so that it may be moved to a dehydration system where it is needed to refine or purify the spent TEG and return it to storage for reuse. The TEG can also be used to clean the dehydrator. Those in the art will understand that the dehydrators become loaded with contaminants which are hazardous, which in turn, requires expensive disposal. However, with this process, the TEG can be circulated through the dehydrator system and greatly reduce the volume of material requiring disposal.
For background information relating to glycol dehydration systems for treating natural gas, reference may be had to U.S. Pat. Nos. 5,163,981; 5,116,393; 4,375,977 and 4,661,130.
The problems encountered with TEG dehydrators is that along with water, the TEG starts to pick up small amounts of light liquid hydrocarbons. These hydrocarbons are not as easily removed as the water in the regeneration phase and a certain amount of the hydrocarbons remain with the lean TEG as it circulates back through the absorber column. The hydrocarbons attract other contaminants along with more hydrocarbons that are found in the gas stream and the TEG becomes further diluted with pollutants. As the TEG becomes more saturated, contaminants and hydrocarbons are increasingly more difficult to remove in the regeneration process. Some aromatic hydrocarbons are passed along with the water vapor into the atmosphere. These aromatic hydrocarbons are considered pollutants. They include benzene, toluene, ethylene and xylene, commonly known as BTEX. They are environmentally hazardous and considered carcinogens. These and other hydrocarbons that may be generated in the process of dehydrating natural gas are referred to as volatile organic compounds(VOC). The control of BTEX and other VOC emissions from TEG dehydration units is of increasing concern to environmental protection both at the federal and state levels. Air quality regulations in the United States are increasing because of the Clean Air Act Amendments (CAAA) of 1990. Other regulations include the National Emission Standard Hazardous Air Pollutants (NESHAP) program and state regulatory agencies.
Another source of contamination to the TEG system is salts. Carry over of brine solutions from the field can lead to salt contamination in the TEG system. Sodium salts (typically sodium chloride) are a source of problems in the reboiler since sodium chloride is less soluble in hot TEG than in cool TEG. Salts will precipitate from the solution at typical reboiler temperatures of 350 to 400 degrees Fahrenheit at atmospheric pressure. The salt can deposit on the fire tube restricting heat transfer, causing the temperature of the fire tube increase, which will lead to thermal degradation of the TEG. The salt will also increase corrosion of the fire tube. The dissolved salts cannot be removed by mechanical filtration. When the salt content reaches 1% the TEG is spent and should be reclaimed or replaced.
After a period of time, the TEG becomes severely contaminated and loses its effectiveness as a desiccant and is considered xe2x80x9cspentxe2x80x9d. TEG at 94% concentration in solution becomes increasingly ineffective as a desiccant. The presence of contaminants may result in fouled equipment, foaming, poor dehydration and the potential of increased release of pollutants into the atmosphere. The options for spent TEG are to dispose of it and replace it with new TEG. The spent TEG may also be sent to a reclaimer for recovery. (Both of which are not very economic and will require the dehydration system to be shutdown.) During these down times operators currently choose to clean the dehydration system at considerable costs, which produces large amounts of hazardous waste to be disposed of. This also increases the chance of spilling the hazardous waste onto the ground causing more problems.
(1) Progressive Contribution to the Art
According to this invention, the glycols may be cleaned and recycled by vacuum distillation. The glycols have an evaporation temperature at atmospheric pressure higher than the temperature at which they degenerate. It is necessary to evaporate them at a temperature below the point that they degenerate. The evaporation temperature is elevated as high as possible but still must be below the degeneration point. The absolute pressure is reduced on the evaporating liquids on a economy basis. The absolute pressure is reduced as low as possible for rapid evaporation and temperatures no higher than necessary. However, to obtain extremely low absolute pressures is difficult and expensive. Therefore the pressure used for evaporation is one that is a balance between these considerations.
An improved process of refining TEG is provided in which the unit is a mobile self contained unit that will purify the spent TEG by substantially reducing the amount of entrained solids and hydrocarbons. The refining unit can also be used to clean contacting towers, heat exchangers, pumps, still column, reboiler and surge tanks of TEG dehydrators. This will provide a 97% volume reduction in hazardous wastes compared to conventional methods of cleaning dehydrators. The unit accomplishes this with vacuum distillation incorporating a closed system. The refining unit utilizes the TEG located at the site along with a chemical degreaser that aids in the removal of coke and sludge buildups. All contaminants and foulants that are removed from the contacting tower of the dehydration system are removed by the refining unit. The unit eliminates the risk of hazardous waste spills that commonly occur with conventional cleaning methods of dehydration systems. All wastes that occur during cleaning and refining are gathered in the refining unit. These wastes can be collected very easily to be disposed of properly. The refining unit will provide efficient cleaning of dehydration equipment, reduce hazardous waste volume, convert hazardous waste into a useful product, reduce the emission of dangerous VOC""s and BTEX all without shutting in gas sales. The refining unit is able to accomplish this since it is a closed system. The unit can utilize dehydrated gas or LPG at the site for fuel gas for the burner, power medium used in the operation of valves, shutdowns and fluid transfer.
The refining unit is kind to the environment by utilizing spent solution in a cleaning process, returning a spent solution to its usable form, recycling spent solution which eliminates the need to produce more solution, eliminating the emission of hazardous VOC""s and BTEX during refining, reducing the emission of VOC""s and BTEX in TEG dehydration units, reducing hazardous waste that can be created during a cleaning operation, eliminating the risk of soil contamination due to hazardous spills, simplifies the collection of hazardous waste at the site and accomplishing all of the aforementioned while the dehydration system continues gas sales.
This invention also is used to recycle glycols used as anti-freeze. It is more economical to gather the used anti-freeze and transport it to a stationary recycling plant.
This invention reduces if not completely prevents the release of BTEX gases into the atmosphere. This invention reduces by 97% the amount of hazardous waste from the polluted glycol. It eliminates the added industrial hazard of the conventional cleaning of the polluted gas plant equipment. The normal cleaning of the equipment greatly increases the volume of the hazardous waste; making up to five times as much hazardous waste as originally present because of the volume of the polluted cleaning fluid. The immediate vicinity is protected by the use of a completely closed system.
This process has the following unique features and advantages:
(1) To purify the TEG with the unit requires only one cycle.
(2) Self containment of the TEG refining process, hydrocarbon adsorption process and dehydration system cleaning process, thus the equipment of the refining unit may be mobile.
(3) Process that reduces a contaminated waste product to the lowest possible volume for disposal while purifying TEG and cleaning dehydration systems.
(4) Recycles contaminated TEG instead of the disposing of contaminated TEG and replacing with new TEG at approximately 50% of the replacement costs.
(5) Reduces disposal of hazardous waste by over 95%.
(6) Allows user to produce natural gas uninterrupted while purifying TEG in the refining unit.
(7) Allows user to produce natural gas uninterrupted while cleaning the dehydration system.
(8) Reduces waste disposal generated while cleaning dehydration system from 100% to 1% by volume.
(9) Does not emit hazardous VOC""s and BTEX.
(10) Provides the process in closed system eliminating the risk of hazardous waste spills.
(11) Helps reduce hazardous air emissions from dehydrator units.
(12) Improves gas/glycol contacting resulting in better water removal.
(13) Eliminates foaming stopping uncontrolled glycol losses.
(14) Reduces the risk of salt contamination in the dehydration system which can cause hot spots on the fire tube, thermal degradation of the TEG and increase corrosion in the dehydrator system.
(2) Objects of this Invention
An object of this invention is to recycle glycols for reuse.
Another object of this invention is to recycle glycol from a natural gas dehydration plant and simultaneously clean the equipment of the natural gas plant to both rejuvenate the glycol and to concentrate and remove the hazardous waste from the natural gas plant.
A further object of this invention is to recycle glycols used as anti-freeze.
Further objects are to achieve the above with devices that are sturdy, compact, durable, lightweight, mobile, simple, safe, efficient, versatile, ecologically compatible, energy conserving, and reliable, yet inexpensive and easy to manufacture, install, operate, and maintain.
Other objects are to achieve the above with a method that is rapid, versatile, ecologically compatible, energy conserving, efficient, and inexpensive, and does not require highly skilled people to install, operate, and maintain.
The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawings, the different views of which are not necessarily scale drawings.